Inorganic pigments, such as calcium carbonate pigment particles, are often dispersed in a solvent, such as water, to form a liquid coating that when applied to a substrate, such as paper, and subsequently dried forms an image-receiving layer. A binder is usually added to the liquid coating to adhere the calcium carbonate particles to each other and to the substrate after the coating is dried. The calcium carbonate particles produce a glossy layer with absorption suitable for inkjet printing.
The liquid coating is often formed from an anionic dispersion (or slurry) of ground or precipitated calcium carbonate pigment particles. However, inkjet ink is typically anionic so inkjet ink has trouble adhering to an image-receiving layer made from an anionic dispersion of calcium carbonate pigment particles. Therefore, the anionic dispersion of calcium carbonate pigment particles is typically converted to a cationic dispersion by adding an excess of highly charged cationic dispersant to the anionic dispersion of calcium carbonate pigment particles. During the process of charge conversion, however, the viscosity of the dispersion may increase and flocculation may occur.
Calcium carbonate pigment particles, either with a cationic or anionic charge, are often too large, as received, for producing gloss levels suitable for photographic printing, such as can be obtained using fumed or gelled alumina or silica. Therefore, the calcium carbonate pigment particles are often milled, in slurry form, to produce sufficiently small particles. Before applying the slurry of ground calcium carbonate pigment particles to a substrate to form an image-receiving layer, a binder is usually added to the slurry of ground calcium carbonate pigment particles, as a separate process step, so that the calcium carbonate particles adhere each other and to the substrate after the slurry is dried.